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Building from a pair of MA-9S2 boards, first project
Building from a pair of MA-9S2 boards, first project
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Old 11th April 2016, 01:33 AM   #11
cut is offline cut
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Join Date: Dec 2013
Thanks! I think that my predicament is pretty much precisely because there is so much information and discussion available. The design I'm working on attempts to be my synthesis based on what I found which seemed relevant, though I am sufficiently a novice that I would love it were to someone take a look and point out obvious deficiencies or elements which may have arisen from my misunderstandings.

I'll go read that article for sure.
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Old 11th April 2016, 07:55 AM   #12
Ian Finch is offline Ian Finch  Australia
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Building from a pair of MA-9S2 boards, first project
OK, if you are uncertain what to do now but you've decided you need to start with a basic power supply built from parts you source individually, as your schematic suggests, you need to decide a few things like the numbers that will set the limits for your end result.

First though, if you check your schematic against other dual rail supplies, you'll notice you have used further split supplies for each rail, so much of what you have drawn is it is unnecessary. This circuit below isn't exactly what I mean but it's close enough to illustrate the elements and all you need for a basic dual bridge, dual rail supply.
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A 2 x 25V AC transformer means about 35V DC power rails and that limits the amplifier to perhaps 50W/8R output. Going a little higher to a 2 x 35VAC transformer, gives about 50VDC and 100W/8R. No point going much higher unless your speakers are thirsty monsters and your room is large. A rule of thumb for a single toroidal transformer rating in VA, is 4 X the nominal 8R output power per stereo channel. So if you expect 100W/8R and 150W 4R, you need approximately a 400VA toroid. You could use less, like a standard 300VA transformer, assuming you don't use the amplifier at maximum power levels for more than transient signal peaks. Any diode bridges from 10-50A and 200PIV rated will be fine there. The smoothing caps should be 10,000 - 15,000 uF 63V and the bleed resistors larger, at about 2k 2W, IMV.
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Old 9th July 2016, 09:31 AM   #13
bjed is offline bjed
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Default Marantz power ampifier MA-9S2 - BIOS

Hello
Does anyone have an idea of the final transistors BIOS current ?

//Bjorn
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Old 23rd September 2016, 09:40 PM   #14
carlmart is offline carlmart  Brazil
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Have you built this kit?
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Old 24th September 2016, 12:37 PM   #15
esgigt is offline esgigt  Netherlands
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Quote:
Originally Posted by bjed View Post
Hello
Does anyone have an idea of the final transistors BIOS current ?

//Bjorn
BIAS current!
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Old 12th December 2016, 11:44 PM   #16
chalky is offline chalky
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The sellers say 50-100mA per output transistor ( 10 - 40mV across any 0.22R emitter resistor ). Seems a bit high to me - min distortion is usually 10-15mA per device for class AB. I've built up a couple of these pcbs and will eventually set them up for min thd with my spectrum analyzer. The circuit of the "clone" amps, as mentioned previously, is used by Marantz simply as a low gain output buffer, with voltage amplification provide elsewhere. All of the open loop gain of the "clone" is provided by the two cascode transistors and is necessarily quite low when compared with the open loop gain of, say, a Self blameless amplifier. The upshot of this is higher distortion than that produced by a Self blameless. A possible solution to this problem is to set the gain of the "clone" to 5 - 10 and precede it with a decent opamp ( eg LME49710 ), included within the overall feedback loop to give a final gain of 20 - 30. I'll probably try this sometime.
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Old 13th December 2016, 06:36 AM   #17
Ian Finch is offline Ian Finch  Australia
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Building from a pair of MA-9S2 boards, first project
I assume the seller really means to say "50-100mA per complementary pair" of output transistors. The clone is only a 2-pair EF2 design, unlike the original design. It probably comes with copies of the original transistors or the price would be inflated to NOS level.

50 mA/pair could be sufficient to minimise crossover distortion with genuine original output stage transistors but copies or fakes will probably need more, as the seller suggests by allowing for 100mA maximum. Older, plain epitaxial transistor types in copied cases could need a full 100 mA bias per pair, so that adds up to 100-200ma for each amplifier or 200-400 for the stereo pair. Good luck with the build(s).

BTW, a CFP output stage design can indeed use only 10-20mA bias per pair, but that's a different matter .

Last edited by Ian Finch; 13th December 2016 at 06:47 AM.
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Old 7th November 2020, 09:25 PM   #18
kozard is online now kozard
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Originally Posted by sajti View Post
Hi,

the most important difference is, that Marantz use separated voltage amplifier, and power buffer stage to eliminate the back EMF distortion as much as possible.
So MA9 (and even SM11, and PM11) has two individually feedbacked stages. The output stage has a gain about 10-12dB only. The mentioned clone has the gain of 30.
I made the MA-9S2 kit and I like it quite a bit. For a fun project I was thinking of building another with the gain reduced to 10 dB and making up the gain with one of the clone Marantz JC-2 pre-amplifiers.

I think the gain is set with the ratio of the 2200 and 75 Ohm resistors. Perhaps I should use 2200 and 240 Ohms?

What do you think? Does this sound like a reasonable project and approach?

Perhaps it will get a little closer to the original? I can not do much about the kit not being balanced. But perhaps this proposal brings the gain and number of stages much closer to the original?

Last edited by kozard; 7th November 2020 at 09:29 PM.
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Old 8th November 2020, 05:29 AM   #19
sajti is offline sajti  Hungary
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Not as easy. It may oscillate, because the compensation was planned for higher gain.
Some simulation may help to check, if it is possible.
Sajti
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Old 8th November 2020, 01:01 PM   #20
kozard is online now kozard
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Originally Posted by sajti View Post
Not as easy. It may oscillate, because the compensation was planned for higher gain.
Some simulation may help to check, if it is possible.

Sajti

Yes, that is a real concern.

I was wondering how the MA-9S2 DIY compensation was done.

All I see as possible compensation components are the 10 Ohm and 220 pF which I have marked in the enclosed schematic and layout. Am I correct? Are those the components used for compensation in this design?

I wonder if anyone has a better schematic for this. I wonder if anyone has done any simulations on this design?
Attached Images
File Type: png How is MA-9S2 DIY Compensated Schematic.png (337.8 KB, 71 views)
File Type: jpg How is MA-9S2 DIY Compensated PCB.jpg (199.2 KB, 75 views)
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